Various information recording techniques have been developed along with recent increase of capacity in information processing. Particularly, the areal recording density of hard disk drives (HDDs) using a magnetic recording technique has continued to increase at an annual rate of about 100%. Recently, a 2.5-inch magnetic disk used for an HDD or the like has been required to have an information storage capacity greater than 250 Gbytes per disk. In order to meet such a demand, it is necessary to achieve an information recording density greater than 400 Gbits/inch2. In order to achieve a high recording density of a magnetic disk used for an HDD or the like, it is necessary to reduce the size of magnetic crystal grains that constitute a magnetic recording layer, which is used to record informational signals, and to reduce the thickness of the magnetic recording layer. As a result of development in reduction of the size of magnetic crystal grains, however, the thermal stability of recorded signals is deteriorated due to the superparamagnetism phenomenon in an in-plane magnetic recording type magnetic disk, which has heretofore been commercialized. Therefore, the recorded signals are lost, and a thermal fluctuation phenomenon occurs. Those problems inhibit an increase of the recording density of a magnetic disk. The in-plane magnetic recording type is also referred to as a longitudinal magnetic recording type or a horizontal magnetic recording type.
In order to eliminate those inhibitors, a perpendicular magnetic recording type magnetic disk has been proposed in recent years. Unlike an in-plane magnetic recording type magnetic disk, a perpendicular magnetic recording type magnetic disk has a principal recording layer with an easy axis oriented perpendicular to a surface of a substrate. The perpendicular magnetic recording type can reduce a thermal fluctuation phenomenon as compared to the in-plane magnetic recording type because a recording layer can have a large film thickness. Furthermore, the perpendicular magnetic recording medium has a soft magnetic layer. A magnetic flux from a recording head can converge with the soft magnetic layer. Additionally, a magnetic field larger and steeper than that of a longitudinal magnetic recording type can be generated by the mirror image effect. Accordingly, the perpendicular magnetic recording type is suitable to increase the recording density.
If a magnetic recording layer of the perpendicular magnetic recording type has a hexagonal close-packed (hcp) structure, the c-axis becomes an easy axis. Thus, the c-axis should be oriented in the normal direction of a substrate. In order to improve the orientation of the c-axis, it is effective to provide a nonmagnetic underlayer having an hcp structure underneath the magnetic recording layer. It has been known that such an underlayer can employ CoCr alloy, Ti, V, Zr, Hf, and the like. Among other things, it has been known that ruthenium (Ru) can effectively improve the crystal orientation of a magnetic recording layer and can increase the magnetic coercive force (Hc).
In order to improve the signal-to-noise ratio (S/N ratio) of a perpendicular magnetic recording type medium, magnetic grains of a magnetic recording layer are isolated and reduced in size by forming a granular structure in which nonmagnetic substance (e.g., chromium) or oxide is segregated between the magnetic grains so as to form grain boundaries (see, e.g., JP-A-2003-36525). Furthermore, a magnetic layer is multilayered in order to promote segregation of the nonmagnetic substance or oxide at the grain boundaries and to promote reduction of the size of the magnetic grains (see, e.g., JP-A-2006286106).
Meanwhile, there has been demanded to further increase the information recording density of a magnetic disk. Recently, a 2.5-inch magnetic disk used for an HDD or the like has been required to have an information storage capacity greater than 250 Gbytes per disk. In order to meet such a demand, it is necessary to achieve an information recording density greater than 400 Gbits/inch2, In order to achieve a high recording density of a magnetic disk used for an HDD or the like, it is necessary to reduce the size of magnetic grains and to improve the S/N ratio. In a perpendicular magnetic recording medium with conventional configuration, while a magnetic layer is multilayered, a large amount of nonmagnetic substance (e.g., chromium) is added to reduce the saturation magnetization (Ms) for thereby reducing noise. See, JPA-2006-351055. With this method, the crystal magnetic anisotropy (Ku) of the magnetic layer is lowered so that the magnetic energy of the magnetic layer becomes unable to overcome the demagnetizing field and the thermal energy. Therefore, a thermal fluctuation phenomenon becomes problematic. Additionally, with this configuration, the film thickness of the magnetic layer should be increased in order to output signals. Therefore, an extra material is needed to produce a medium, resulting in an increase of material cost.